The present application relates generally to the use of liquid desiccant membrane modules to dehumidify and cool an air stream entering a space. More specifically, the application relates to the use of micro-porous membranes to separate the liquid desiccant from the air stream wherein the fluid streams (air, heat transfer fluids, and liquid desiccants) are made to flow turbulently so that high heat and moisture transfer rates between the fluids can occur. The application further relates to the application of such membrane modules to locally dehumidify spaces in buildings with the support of external cooling and heating sources by placing the membrane modules in or near suspended ceilings.
Liquid desiccants have been used in parallel to conventional vapor compression HVAC equipment to help reduce humidity in spaces, particularly in spaces that either require large amounts of outdoor air or that have large humidity loads inside the building space itself. Humid climates, such as for example Miami, Fla. require a large amount of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Conventional vapor compression systems have only a limited ability to dehumidify and tend to overcool the air, oftentimes requiring energy intensive reheat systems, which significantly increases the overall energy costs because reheat adds an additional heat-load to the cooling coil or reduces the net-cooling provided to the space. Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as solutions of LiCl, LiBr or CaCl2 and water. Such brines are strongly corrosive, even in small quantities, so numerous attempts have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. One approach—generally categorized as closed desiccant systems—is commonly used in equipment dubbed absorption chillers, places the brine in a vacuum vessel which then contains the desiccant. Since the air is not directly exposed to the desiccant, such systems do not have any risk of carry-over of desiccant particles to the supply air stream. Absorption chillers however tend to be expensive both in terms of first cost and maintenance costs. Open desiccant systems allow a direct contact between the air stream and the desiccant, generally by flowing the desiccant over a packed bed similar to those used in cooling towers. Such packed bed systems suffer from other disadvantages besides still having a carry-over risk: the high resistance of the packed bed to the air stream results in larger fan power and pressure drops across the packed bed, thus requiring more energy. Furthermore, the dehumidification process is adiabatic, since the heat of condensation that is released during the absorption of water vapor into the desiccant has no place to go. As a result both the desiccant and the air stream are heated by the release of the heat of condensation. This results in a warm, dry air stream where a cool dry air stream was desired, necessitating the need for a post-dehumidification cooling coil. Warmer desiccant is also exponentially less effective at absorbing water vapor, which forces the system to supply much larger quantities of desiccant to the packed bed which in turn requires larger desiccant pump power, since the desiccant is doing double duty as a desiccant as well as a heat transfer fluid. The larger desiccant flooding rate also results in an increased risk of desiccant carryover. Generally air flow rates in open desiccant systems need to be kept well below the turbulent region (at Reynolds numbers of less than ˜2,400) to prevent carry-over of desiccant to the air stream.
Modern multi-story buildings typically separate the outside air supply that is required for occupant comfort as well as air quality concerns from the sensible cooling or heating that is also required to keep the space at a required temperature. Oftentimes in such buildings the outside air is provided by a duct system in a suspended ceiling to each and every space from a central outside air handling unit. The outside air handling unit dehumidifies and cools the air, typically to a temperature slightly below room neutral temperatures (65-70F) and a relative humidity level of about 50% and delivers the treated outside air to each space. In addition, in each space one or more fan-coil units (often called Variable Air Volume units) are installed that remove some air from the space, lead it through a water cooled or heated coils and bring it back into the space.
Between the outside air handling unit and the fan-coil units, the space conditions can usually be maintained at proper levels. However, it is well possible that in certain conditions, for example if outside air humidity is high, or if a significant amount of humidity is created within the space or if windows are opened allowing for excess air to enter the space, the humidity in the space raises to the point where the fan-coil in the suspended ceiling starts to condense water on the cold surfaces of the coil, leading to potential water damage and mold growth. Generally condensation in a ceiling mounted fan-coil is undesirable for that reason.
There thus remains a need for a system that provides a cost efficient, manufacturable and thermally efficient method to capture moisture from an air stream in a ceiling location, while simultaneously cooling such an air stream and while also eliminating the risk of condensation of such an air stream on cold surfaces. Furthermore such a system needs to be compatible with existing building infrastructure and physical sizes need to be comparable to existing fan-coil units.